This invention relates generally to filters for filtering and separating fluids. More particularly, the present invention relates to fuel filters for removing foreign particles and separating water from the fuel of the fuel supply system of an internal combustion engine.
Diesel fuel supplies frequently contain significant quantities of abrasive particles and water which present the potential for permanent damage to the components of the fuel injection pump, the fuel delivery system and the engine. Consequently, an effective fuel filter as a practical necessity is conventionally incorporated into the fuel supply system of a diesel engine. A multitude of conventional fuel filters employ a disposable filter cartridge which is replaced at pre-established intervals of filter usage. Such fuel filters perform the dual function of removing particulate material from the diesel fuel and separating water from the fuel.
U.S. Pat. No. 4,976,852 and U.S. Pat. No. 5,084,170, which are assigned to the assignee of the present invention, disclose fuel filter assemblies to which the present invention relates. The fuel filters employ a base which mounts a disposable filter cartridge. In some disclosed embodiments, the cartridge includes a single stage filter system wherein fuel flows axially and radially to a filter element for removing particulate matter. The filter element also functions as a water barrier. Filtered fuel flows axially and exits through an outlet passage of the base. The water may be collected in a sump and periodically removed. The cartridge is secured to the base by a collar which engages against a roll seam structure of the cartridge.
For most, if not all diesel fuel delivery systems, as fuel circulates through the fuel filter, air pockets are formed in the fuel filter. The air pockets normally form at the top of the fuel filter. Air bubbles tend to form in the fuel as a result of gassing when the fuel is exposed to changes in pressure. For pressurized fuel delivery systems, the air pockets tend to disintegrate over time and/or the stream of bubbles present in the fuel is of a relatively small magnitude. The air pockets generally do not result in the formation of large air bubbles in the fuel supply partly due to the vapor pressure of the pressurized fuel. Consequently, for pressurized systems, air pockets in the fuel filter do not present a significant problem or obstacle to the circulation of fuel through the fuel filter and the delivery of fuel to the engine.
However, in vacuum applications, the formation of air pockets in the fuel circulating through the filter can present a more significant problem. The air pockets are much more resistant to absorption over time in vacuum systems. Moreover, the vacuum suction exerted may result in the formation of tiny air bubbles or "gassing" in the fuel. This is especially true as the filter element approaches the end of its useful life and the pressure drop across the filter media increases. This increasing restriction to flow causes more air to be pulled from the fuel as the differential pressure between the clear side and the dirty side increases. In of themselves, these individual tiny bubbles do not present a problem to normal vehicle operation as they are easily ingested by the fuel injection pump. When subjected to pump charging pressures, the tiny bubbles are ordinarily reabsorbed by the fuel.
The problems in vacuum suction systems initially arise on the clean side of the filter element where the small bubbles inevitably collect, agglomerate and form air pockets, e.g., large bubbles. The large bubble formation will continue over a period of time until the physical attitude of the filter element changes due to vehicle maneuvering, etc. When these relative large air pockets or bubbles escape into the fuel flow, they are not easily ingested and may cause engine operating problems such as stalling or engine misfire.